Method of manufacture of silicone rubber



Oct. 15, 1963 H. T. STEWART METHOD OF MANUFACTURE OF SILICONE RUBBER Filed Oct. 10. 1960 2 eets-Sh 1 ENmR. Hugh Slewarr Oct. 15, 1963 H. 'r. STEWART METHQD OF MANUFACTURE OF SILICONE RUBBER Filed Oct. 10. 1960 2 Sheets-Sheet 2 L0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 l0.0 RCENT 0F CUMULATIVE WEIGHT LOSS- m9876= um3m w9 8765432 0 w ImmE m0 mmDOI lE-Z INVENTOR.

HUGH T. STEWART ATTORNEY United States Patent 3,107,234 METHOD GF MANUFACTURE OF SILICONE silicon. In the early :Work with silicon rubber, and to a great extent as of today, these hydrocarbon radicals were generally the methyl group (CH and such material is referred to as dimethyl polysiloxane. the following recurring structure:

Thus, in the Patents No. 2,709,161 to Kilbourne and r 0. 2,816,089 to Willis, filed in the raw gum material which was vulcanized Was dimethyl polysiloxanc.

Company), phenyl (General Electric Company), methyl trifiuoro '(Dow Corning Company), methyl vinyl (General Electric Company) and methyl (General Electric Company). And a paper entitled Properties and Uses of a New Vinyl-Containing Silicone Rubber, bearing the date May 16, 1956, Which Was prepared by the Development Laboratories of Union Carbide and Carbon Corporation and presented before the as vinyl, phenyl, etc., the said other radicals are substituted for some of the methyl radicals that constitute phenyl vinyl 'ployed to subject the material undergoing curing 3,107,234 Patented Oct. 15, 1963 silicon atoms present in the molecule. The authorities and references from among the commercial suppliers of raw silicone gum for fabrication into are in agreement that in the case of phenyl groups the substitution is in the range of 5.0 to 10 case of vinyl The phenyl substitution is believed to impart resistance to low temof the vulphenyl vinyl polysiloxanes are also available. In fact, this and the methyl vinyl gum above referred to are the only two vinyl-containing polysrloxanes that are at this time commercially avail- Throughout the specification and claims of this ap- Whenever used, is product produced by the vulcanization of polysiloxanes, such as those here- It is a term Well recognized and to designate such the term silicone resin, which latter term is Well recognized as designating rigid, non-elastomeric product made from certain polysiloxanes. is regularly used throughout the trade to designate those polysiloxanes inally, the filler is tion of the catalyst so as to prevent any premature curing of the gum on the mill. The compounded material is then subjected to heat in one manner or another to give the final product, a mold being used in the curing and the material has been held in the mold for more or less prolonged periods of time. It has also invariably been the practice in these methods as heretofore emto a prolonged oven-heating post-curing period. Willis, for example, in Patent 2,816,089 hereinabove referred to, kept the material in the mold for one hour at from 275 F. to 350 F. and then oven-cured for from 4 to 48 hours at around 350 F.; and Kilbourne, in the process of his Patent 2,709,161 above referred to, mold-cured for from 15 to 30 minutes at 225 F. to 275 F. and followed this with an oven cure for from 1 to 48 hours at 300 F. to 480 F.

Curing catalysts or agents have generally included certain organic peroxides,

the process of the aforesaid Willis patent.

Filler material is required patent and silicone rubber parts processing characteristics. Fillers are generally grouped into two classes: reinforcing fillers, with which it is possible to make relatively high strength silicone rubbers, i.e., they increase the tensile strength, hardness, etc.; and semior non-reinforcing fillers, with which other properties are obtained with only moderate strength. They are inorganic and of three major types, namely, manufactured silicas, naturally occurring silicas and metallic oxides, such as oxides of Zinc, iron and titanium. 'The greatest reinforcement is obtained with the manufactured silicas. Included in this type, for example, is the Hi-Sil X303 referred to in the examples of the aforesaid Willis stated by Willis to be a silica aerc-gel supplied by the Pittsburgh Plate Glass Company. The metallic oxides produce little or no reinforcement and are used primarily as an extender. The naturally occurring silicas, such as diatomaceous earth, are classified as semi-reinforcing fillers. The difference in physical properties imparted to silicone rubbers by diiferent fillers supplied by the trade is shown by typical tensile strengths published in a Silicone Rubber Handbook put out by General Electric Company. Among the reinforcing fillers, a precipitated silica of one supplier is listed as giving a tensile strength of 600-900; that for a silica aerogel of another supplier, 600-900; for a fumed silica by another supplier, 600-1200; and for a coated silica of still another supplier, 1000-1200. Among the various semi-reinforcing and non-reinforcing fillers of different suppliers a tensile strength range of 400-800 is given for a calcined diatoma ceous silica; 400-800 for a calcined kaolin; 100-400 for ground silicas of different suppliers; 200-500 for Zinc oxide, iron oxide and titanium oxide supplied to the trade; and 400-600 for a Zirconium silicate as supplied to the trade.

While the use of a filler is required to give certain desired properties to the elastomeric product, usties sired properties to the elastomeric product, its use is not an essential element in practicing my invention. A silicone rubber part can be made by practicing the invention without the use of a filler, just as it can in other methods of curing the raw pure gum. e end product would, however, be a fully cured, but quite soft and extensible part; i.e., whereas it could be stretched, it would not completely return to its shape before having been stretched.

It is an object of my invention to provide a cured silicone rubber having the improved physical properties required of rubber parts for industrial use, which include, more specifically, greater Shore hardness, tensile strength, elongation, and less compression set, without the use of any oven post-curing period following the mold set.

Another object is to provide for the production of a cured silicone rubber part by injection molding technique in which the entire curing step takes place in a matter of seconds, thereby permitting curing and molding steps for the. production of a succession of parts, to take place as rapidly as the molding apparatus can be closed and opened for the charging of the raw gum and the-removal of the cured parts. 7 7

Still another object is to accomplish the molding of ch require a high degree of precision of shape and dimensions, with a shrinkage factor during molding substantially as low as that which exists with organic rubbers, thereby rendering it unnecessary to provide molds'which are considerably oversize as has heretofore been the practice. Organic rubbers have a lineal shrinkage in the range of 1.5 to 2.5 percent; whereas, with silicone rubber gums, as heretofore molded, the shrinkage is in the range of to 7 percent, thereby presenting a particularly acute problem where precision of shape and dimensions is important, and necessitating theproviding of oversized molds where undersize in the molded part cannot be tolerated.

These and other objects and advantages I accomplish through the use of a vinyl-containing polysiloxane gum,

2, using as the catalyst a dicumyl peroxide as hereinafter described and in accordance with the present invention as hereinafter described.

My experimentation with polysiloxanes having the vinyl substitution for certain of the methyl groups as hereinabove described, and with those that are not vinyl-containing, as for example, the straight dimethyl polysiloxanes, has strongly indicated that there is a substantial difference in the behavior of the two classes during the curing of the raw gum. For example, with the dimethyl gum, and using first benzoyl peroxide and then dicumyl peroxide as the curing catalyst, I find that by increasing the quantity of catalyst at a given temperature and for the same elapsed time, the amount of cure is increased by increasing the amount of either catalyst. On the other hand, with vinyl-containing gums I do not increase the extent of cure with either catalyst at a given temperature and for the same elapsed time, by increasing the amount of catalyst. Whatever may be the real or complete reason for this difference, I am of the belief that a contributing factor is that with the dimethyl gums having no substituted vinyl radicals the curing tion of dimethylene crosslinks, there being but one crosslink formed for each mole of peroxide decomposed; hence, the greater the amount of either catalyst, the greater the number of dimethylene crosslinks formed and the tighter the cure. On the other hand, vinyl-containing gums cure in other words, by polymerization brought about by the catalytic action of peroxide frag-. ments, with the double bonds present in the vinyl radical.

This fact of an inherent difference between the behavior of vinyl-containing gums and that of other polysiloxane gums is emphasized by a statement in the paper hereinabove refer-red to entitled Properties and Uses of a New Vinyl-Containing Silicone Rubber, prepared 'by Union Carbide and Carbons laboratory. The authors of that paper were dealing with ditertiary butyl peroxide for the curing of a vinyl-containing gum, and they stated that that catalyst can only be used with vinyl-containing gums because it crosslinks only where vinyl groups are present and thus an excess of this catalyst has no effect on the compounds hardness or other physical properties.

The dicumyl peroxides which I employ as the catalyst in curing the vinyl-containing gums are referred to in the Willis Patent 2,816,089, hereinabove referred to, as di(armkyl) peroxides, and are therein described as having the structural formula:

by a chain reaction,

where R is phenyl and R is selected from the group consisting of phenyl and methylphenyl. As is stated in this Willis patent, these peroxides can be prepared by methods known in preparing them involving the condensation of cumene hydroperoxide and an alcohol having the general formula:

where R is the same as in the above for these peroxides. Two of the dicumyl peroxides particularly useful for curing the vinyl-containing polysiloxane gums are bis (a,ot-dimethylbenzyl)- peroxide and a,e-dirnethylbenzyl (a,a-dimethyl-p-methylbenzyl) peroxide.

My experiments have shown that the dicumyl peroxides provide a unique curing combination with the vinyl-containing polysiloxane gums as outlined herein. These catalysts decompose to give only minor amounts of deleterious, volatile by-products which escape from the gum during the curing, thereby giving not only v better control over the cure, but producing more stable cured rubber. This is in contrast to benzoyl peroxide, which l l general formula given brought about is by the formathe art, a particularly convenient method for The proportion of the dicumyl peroxide is pertinent therein. Relatively larger amounts of the catalyst may be used. Generally, owever, I have minimum amount of catalyst which will produce a satisfactory cure will likely give the best end product. With the bis (a,a-dimethylbenzoyl) peroxide, I have noted that evolution of gas, indicating decomposition, begins at around 309 F. I employ a molding temperature in the F. At these temperatures machine can be cycled, i.e., taken through closing the mold, injecting the plastic gu opening the mold and removing the molded The complete cycle from closing to closrunning at about 25 seconds per cycle.

A typical injection molding machine suitable for can-yupon injection of the material for molding, line or mating surfaces 9 are formed with very small air exit openings, such as by placing scratch lines on the mating surfaces. These air discharge openings small to permit the escape of the silicone rubber :ompound but are sufliciently large to permit air evacua- 6 tion of the cavity 8 under the injection pressure which takes place in charging the mold.

The mold sections 6 and 7 at a substantially constant temperature of between 400 F. and 450 F., preferably at about the middle of this range.

The green rubber compound is charged into the mold avity 8 and also carries an axially moves across opening 21 so and compress the green rubber stock in out of the head end of the nozzle 18.

The green rubber stock to be molded may be kneaded or compounded on a conventional rubber mixing mill or reciprocating ram 22 which as to confine sheet or blanket or slab and cut into strips of convenient size for handling and insertion into the bore 20 through opening 21. The standard two-roll rubber mixing mill an imperfect non-filled pant.

As stated hereinabove, my invention will cure the green rubber compound at about the speed that it takes to get the heat through the part in the molding chamber, that is, to bring this is increased, the molding time lengthens because of the thermal insulation properties of the silicone rubber compound. For example, a one-eighth inch section of green silicone rubber compound may be cured by conventional compression molding technique in ten minutes-this being D for 45 F. It will be seen that with the one hour of the standard which has been generally set by the industry. preheat at 300 F. as practiced in the said Kilbourne By way of comparison, a one-eighth inch section may be patent, practically no start whatever toward obtaining a fully cured by the process of the present invent-ion in about stabilized weight is accomplished, the weight loss being five seconds. Actually, as stated hereinabove, it has been only 1.5%. It will also be seen that at the higher prefound that it takes about 20 to 30 seconds to cycle the inheat temperatures employed in my process, substantially jection molding machine, that is, to take it through the all weight loss is obtained in the first few hours, the numseveral steps of opening and closing the mold, removal ber of hours becoming fewer with the higher temperatures,

of the molded part, and charging the injection cylinder there being but little loss in weight after the first four or as required. This normal rapid cycling of the machine 19 live hours with a preheat at 450 F. With my high deatlords a somewhat longer curing time, which is not delgree of preheat, the linear shrinkage during molding beeterious and which permits the molding of somewhat comes substantially as low as that obtained with organic larger sections than the one-eighth inch standard section rubbers- 0 1 the order of 1.5 to 1.8% or thereabouts. discussedwhen the mold is p the moldfid P Where shrinkage is not a factor to be concerned about, and its attached sprue break from the material stream at u h as in th molding of some parts for rough Work, the nozzle Opening near the Cure 131 other Words preheating may, if desired, be omitted. My invention the material in the nozzle remains uncured and breaks may be pracficed either with or Without it. It has now easily at or near the point of engagement of the nozzle become the practice 0f some Suppliers of raw gum to and the adaptor bearing Surface preheat it before putting it out to the molding trade.

My molding system differs from the conventional plastic 20 injection molding system in that the green rubber compound in my process is not heated in the injection cylinder. In the conventional injection molding machine, the granular molding material which is fed into the machine from a hopper is converted to a softened plastic form by means of heat in the injection cylinder, and the hot plastic mass is then injected under pressure into a closed but The following examples are presented showing results obtained in the practice of my invention. The vinyl-containing, vulcanizable polysiloxane raw or uncured gums of these examples were those regularly sold to the trade for the production therefrom of silicone rubber. These gums, as supplied, have various tillers already mixed therein. The suppliers do not regularly pub- -cooled mold where it is hardened by cooling in the mold. fish the kind and amount of finer Contained in the In Certain instances such as Where high precision in but they state the shore hardness and other physical shape and dimension of the molded parts is required, p P which the gums with 11161 miXed fillers making'it desirable to avoid shrinkage during the moldare desig d when Vulcanized fully cllffid- 111 the ing, I may subject the raw gum into which the desired P herewith given, this 511016 hardness rating y filler has been mixed, but prior to the mixing in of the the supplier is given. Similarly, the suppliers of these catalyst, to a preheating period at a temperature in the gums do not give the amount of substitution of vinyl range upwardly from around 375 F. to 500 F., preferor phenyl vinyl radicals tor methyl radicals in the gums y at around 'F- This preheating converts to they supply; but, as hereinabove stated, all suppliers are gasfimls PIOdHQtS, fllfeby PF their escape from in agreement that in order for the gums to cure to siliqg Certain P monfimeficficnstimfifl'is cone rubber-an elastomeric product-the amount of whlch, if allowed to remam 1n the gum will the s such substitution is small-on the order or 0.5 to 1.0

step, would give rise to substantial shrinkage of the material from the mold, a lineal shrinkage in the range of from 5 to 7 percent sometimes resulting. While the ideal amount of preheating is that which will drive out all such gaseous matter, so that the weight of the gum, or the gum plus any filler therein, as the case may be, becomes stable under further heating, I have found that by preheating in the range of the foregoing high temperatures for a period of around 4 or 5 hours, the weight of the mass becomes substantially stable for general use in the fabrication of molded parts. Where extreme precision in shape and dimensions is required, the preheating may be carried further. My preheating is at a very much EXAMPLE 1 higher temperature and for a longer period, than has been employed in the prior art For example Kijboumej 55 Methyl Vznyl Polyszloxane-Suppliers Rating for Hardin Patent 2,709,161 hereinabove referred to, conducted f Cured Rubber Fwm Stock Supplied, 40 a very mild preheating-one hour at 350 F.--his pri- Shore A mary object being to thin, i.e., to plasticize, the mix so as to increase the homogeneity and close contact, i.e., surpercent of the methyl groups for vinyl, and 5.0 to 10.0 percent for phenyl radicals. The vinyl and phenyl content of the gums of these examples is understood to be within these ranges. For purposes of comparing my results with the suppliers ratings of the properties of cured rubber from the raw gums, my molded product Was oven-cured, although oven curing is not generally a part of my process and is not necessary for the carrying out of my process. The comparative data are given in 59 the various examples.

[Catalyst bis a,a-dirnethylbenzyl) peroxide] face wetting, of the pigment (filler) particles by the rubber 60 Amount uncured k 131 0 before the curing agent is added, oth r p s ibl n fi ial Amount catalyst, 0.60% gms 1.09 effects, including the possibility of r du ng h g that Injection mold temperature F 430 might come from such mild preh at g, bei g i fi Time held in mold secs 30 The vast difierence between the approach toward a stabilized weight. (essential for the solving of the shrinkage problem) accomplished by the mild preheating of the prior art and that obtained by my high temperature prei heating is seen from the curves shown in PEG. -2 or" the M su p lier drawings where the percent of cumulative weight loss ob- 7 3, 5 fifi c tained during the preheating of a methyl vinyl polysilox- F ane vulcanizable gum containing less than one percent Properties of product removed from mold:

v t l of vinyl substitution for methyl radicals is plotted against igtifi duration, in hours, of the preheat1ng--curve A for 300 Elongation, percent 540 50 F., curve B for 350 F., curve C for 400 F., and curve Properties after oven post cure for 24 hours at 480 R:

Suppliers V Process rating X R Shore A Hardness 42 40 Tensile Strength 950 825 Elongation, percent 300 375 percent 20 EXAMPLE 2 Methyl Vinyl Polysiloxane-Su'ppliers Rating for Hardness of Cured Rubber From Stock Supplied, 45-55 Shore A [Catalyst bis a,adi.methy1benzyl) peroxide] Amount uncured stock gms 229.2 Amount catalyst, 0.50% gms 1.15 Injection mold temperature F 430 Time held in mold secs 20 Properties of product removed from mold:

Suppliers My rating (15 Process min /250 Shore A Hardness Tensile Strength (psi) Elongation, percent Properties after oven post cure for 24 hours at 480 Supplier's Process rating x Shore A Hardness 51 53 Tensile Strength 1, 020 1020 Elongation, percent 360 340 Compression Set (22 hrs/350 F.), 24

percent 27 EXAMPLE 3 Methyl Vinyl Polysiloxane4uppliefs Rating for Hardness of Cured Rubber From Stock Supplied, 80 Share A [Catalyst bis(a,a-di.methylbenzyl) peroxide] Amount uncured stock gms 231.8 Amount catalyst, 0.10% gms 0.23 Injection mold in temperature F 430 Time held in mold secs Properties of product removed from mold:

Suppliers My rating (15 Process min/250 F.) Shore A Hardness. 79 56 Tensile Strength (psi 1, 020 950 Elongation, percent- Properties after oven post cure for 24 hours at 480 F.:

Y My

Suppliers Process rating Shore A Hardness 82 80 Tensile Strength 920 800 Elongation. percent 230 225 Compression Set i'liiil percent 10 EXAMPLE 4 Methyl Phenyl Vinyl PolysiloxaneStlppliers Rating for Hardness of Cured Rubber From Stock Supplied, 60 ShoreA [Catalyst: bis(a,a-dimethylbenzyl)peroxide] Amount uncured stock gms 226.0 Amount catalyst, 0.20% gms 0.45 Injection mold temperature F 430 Time held in mold secs 25 Properties of product removed from mold:

Suppliers My rating (15 Process min/250 F.) N Shore A Hardness 48 48 Tensile Strength (p.s.i.) l, 560 l, 600 longation, percent 600 600 Properties after oven post cure for 24 hours at 480 F.:

Supplier's Process rating N Shore A Hardness 60- 58 Tensile Strength 1, 420 l, 000 Elongation, percent 420 450 Composition Set percent 20 24 EXAMPLE 5 Methyl Vinyl PolysiloxaneSuppliers Rating for Hardness of Cured Rubber From Stock Supplied, 70 Share A [Catalyst a,a-dimethy1benzyl (a,a-dimethyl-pmethylbenzyl) peroxide] Amount uncured stock gms 228.3 Amount catalyst, 0.20% gms 0.46 Injection mold temperature F 430 Time held in mold secs" 30 Properties of product removed from mold:

EXAMPLE 6 -Methyl Phenyl Vinyl Polysiloxane Suppliers Rating for Hardness Shore A [Catalyst a,'a-dimethylbenzyl a a-dimethy1-pmethylbenzyl) peroxide] of Cured Rubber From Stock Supplied, 50

Amount uncured stock gms 224.2 Amount catalyst, 0.20% gms 0.45 Injection mold temperature F 430 Time held in mold secs 25 Properties of product removed from mold:

Suppliers My Rating Process (15 min F n Shore A Hardness 52 48 Tensile Strength (p.s.i.) 1, 020 900 Elongation, percen 470 450 Properties after oven post cure for 24 hours at 480 F.:

My Suppliers Process Rating f Shore A Hardness 54 52 Tensile Strength 1,150 1, 100 Elongation, percent. 400 400 Compression Set (22 hr cent 15 EXAMPLE 7 Methyl vinyl polysiloxane parts/wt 100.0 Fumed silica do 50.0 Bis(a,u-d-imethyl)peroxide) do 0.04 Injection mold temperature F 425 Time held in mold secs 30 Properties of product removed from mold:

Suppliers Rating (10 min./ 250 F. My with 2,4- Process dichlor benzoyl peroxide 2.00 parts/ Wt. Short A Hardness 52 50 Tensile Strength (p.s 1,020 900 Elongation, percent 330 300 Compression Set (22 hrs/350 F.), percent 25 30 EXAMPLE 8 Methyl phenyl vinyl polysiloxane parts/wt 100.0 Silica aerogel do 40.0 Calcined diatomaceous earth do 20.0 Iron oxide do 1.0 a,a-Dimethylbenzyl(a,a dimethyl-p-methylbenzyl) peroxide "parts/wt 0.40 Injection mold temperature F 430 Time held in mold secs 40 Properties of product removed from mold:

Snppliers ating 0 F. My 7 with Process ditertiary butyl peroxide 2.00 parts] wt. Shore A Hardness '65 65 Tensile Strength (p.s.l) 960 850 Elongation, percent 270 240 (22 hrs/350 F.) 37 4o Compression Set In the following examples the raw gum stock was preheated and then cooled to room temperature before mixing in the catalyst:

1?. EXAMPLE 9 Methyl Vinyl Polysiloxane-Suppliers Rating for Hardness of Cured Rubber From Stock Supplied, 40 Shore A [Catalyst urn-dimethylbenzyl (-a,a-dimethyl-pmethylbenzyl) peroxide] Weight before preheating gms 250.0 Weight after 4 hrs/450 F. gms 227.9 grrrs 22.1

r t 1 Loss during p ehea 11g Lnpercentul 8.84 Amount of catalyst, 0.10% gms 0.23 Injection mold temperature F 430 Time held in mold secs 25 Properties of product removed from mold:

Suppliers My rating (15 Process ruin/250 F. Shore A Hardness 39 37 Tensile Strength (p.s.i.) 1, 090 950 Elongation, percent. 540 500 Linear Shrinkage 1. B

Properties after oven post cure for 24 hours at 480 F.: Suppliers My rat g (15 Process min/250 F.) Shore A Hardness V 42 Tensile Strength 920 825 Elongation, percent 300 375 Compression Set (22hrs.l350 E), percent. 17 20 Linear shrinkage 1. 8

EXAMPLE 1O [Catalyst 1 bis (u,a-dimethylbenzyl) peroxide] Weight before preheating gms.. 250.0 f Weight after 8 hrs/500 F gms 235.0 v gms 15.0 Loss during preheat percent 6 Amount catalyst, 1.0% gms 2.35 Injection mold temperature F 400 Time in mold se,cs 60 Properties of product removed from mold:

S l'e My $83565 Process min/250 M Shore A Hardness 57 45 Tensile Strength (p.s.i.) 1, 660 1, 600 Elongation, percent 840 850 Linear Shrinkage 2.3

Properties after oven post cure for 24 hours at 480 Suppliers My rating (10 Process min/250 M Shore A Hardness 60 60 Tensile Strength 910 700 Elongation, percent 310 250 Compression set (22 hrs 50 13 percent. 42 60 Linear Shrinkage 2. 3

It will be understood that the examples given hereinabove are exemplary in nature, that various changes and modifications therein may be made without departing from the spirit of the invention, and that there is com prehended in the invention such, modifications as come within the scope of the following claims.

3,107,234 13 I claim: vulcanizable polysiloxane; admixing therewith an inorl. The process of making an elastomeric silicone rubganic filler; heating said mixture in the absence of a her part which consists in selecting a vinyl-containing curing catalyst at from about 375 F. to 500 until its 0.1% to 1.0% and having the structural formula oxide in an amount ranging from about 0.1% to 1.0% CH3 CH3 and having the structural formula I R(|3OO-CR 3H3 3H3 CH; OH; 10 R-('3O-OCR1 where R is phenyl and R is selected from the group con- CH3 CH3 substantially closed mold pre-heated to about 375 F. to mass; charging said mass at relatively high speed into a 450 F. to define a part; retaining said part in said mold 15 substantially closed mold pro-heated to about 375 to for a period of time sufiicient to effect cross-linking of the 450 F. to define a part; retaining said part in said mold vmyl groups of the molecule prior to cross-linking of the for a period of time sufiicient to effect cross-linking of methyl groups of the molecule, and opening said mold and the vinyl groups of the molecule prior to cross-linking of removing said part. the methyl groups of the molecule, and opening said mold e process of claim 1 wherein said plastic mass is 20 and removing said part. charged into said mold at high speed so as to fill said mold 5. The process of claim 1 wherein said dicumyl percavity before any substantial curing or vulcanization of oxide is bis (a,a-din1ethylbenzyl)peroxide. said mass takes place. 6. The process of claim 1 wherein said dicumyl per- 3. The process of claim 1 wherein said plastic mass is id i D a-difllfithYlbBflZYl(a,c(-dimethyl p h lb charged into said mold at high speed so as to fill said mold 2v l) e gxjd of said mass tkes P and Said P is retained in said References Cited in the file of this patent UNITED STATES PATENTS her part which consists in providing a vinyl-containing 30 2,816,089 Willis Dec. 10, 1957 

1. THE PROCESS OF MAKING AN ELASTOMERIC SILICONE RUBBER PART WHICH CONSISTS IN SELECTING A VINYL-CONTAINING VULCANIZABLE POLYSILOXANE; ADMIXING SAID POLYSILOXANE WITH A DICUMYL PEROXIDE IN AN AMOUNT RANGING FROM ABOUT 0.1% TO 1.0% AND HAVING THE STRUCTURAL FORMULA 